“Secure Flight is a risk-based passenger prescreening program that enhances security by identifying low and high-risk passengers before they arrive at the airport by matching their names against trusted traveler lists and watchlists,” England told HuffPost.

CLARIFICATION: A previous version of this story said the “SSSS” code came into use in 2010. While that was when the TSA’s Secure Flight program began using the code, the TSA says another government program may have been using it before that, as travelers reported seeing it years earlier.